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Questions About the Mini-Whip Antenna

Hello,
I recently installed a Mini-Whip (Build by RA0SMS) and a KiwiSDR at home (http://redqueenuy.ddns.net:8073/). I live near powerful MW broadcasters, so I added a band-stop filter to avoid having over-voltage in the ADC. For the KiwiSDR, I'm using a linear power supply, ferrite in all the input cables and I reduced the Ethernet adapter speed to avoid introducing noise.

Then I'm using a Mini-Whip by RA0SMS. The antenna is grounded (to a house metal pipe) via the GND connector, and right now isn't very high since is on a balcony and not in the rooftop. I'm using a super cheap 20m long RG58 since I read that the cable isn't that important in this kind of antennas, but now I'm not so sure. I been reading a lot of articles about the Mini-Whip, and would like to ask you guys (that have more experience using these antennas) the following questions:

1) Is the cable important in this case?
Should I replace the cheap RG58 for a more expensive version? Should I use RG213 instead or other kind of cable?
Is the cheap RG58 picking electric noise? Should I use a shielded cable?

2) Is the cable part of the antenna?
Running the cable a few meters in a horizontal position would help receiving horizontally polarized transmissions?

3) Is a linear power supply vital for this antenna?
I'm using an adjustable (laboratory equipment) switching power supply. I ran an experiment by replacing it with a 9V battery and didn't change much (no decrease in the noise floor or less spurs).

4) Is ASL vital for this antenna?
Would it help me put the antenna in the rooftop (3m high)? Right now I'm using a 3m long dielectric mast but is on a balcony.

5) Should I add another ground point?
I discovered when I touch the coax near the antenna, the noise decreases a bit. That's why I made coax choke using a PVC pipe, but I don't think this is optimal.

6) Which voltage are you using?
I'm using 5.0 volts. I been experimenting and found out that I get a decent SNR around that voltage.


Remember that I live in a city near TV, FM, AM and other kinds of transmitters and noise sources (like led lights, switching mode power supplies, transformers, etc).
I might not be able to get rid of all interference.

Thanks!
CX8BIT

Comments

  • edited December 2019
    1. Yes, Mini whips can pick up a lot of noise on the feed, experiment, satellite coax like WF100 has great shielding at low cost, before I moved to WF100 generally I used coax (RG58) vertically down (7m) from the antenna then CCTV balun and CAT5 back to the house and another CCTV balun, noise here is severe. WF100 with F-type to BNC adapters is used here as the RX feed other than external CAT5 shielded.
    2. The cable will feed common mode noise to the house and antenna. In my limited experience electrical probes are not the best choice for city locations unless you can get good separation, use a mag loop instead. I don't personally think a horizontal section will make a positive difference when you consider the wavelengths, but if it being horizontal also includes more distance from noise sources give it a go.
    3. Linear is best, then play with voltage as you have, as it only needs a small supply so look at what people are throwing out, even ask older folk if they have some in the drawer*, gather linear psu's and test them for this use, some are better, some have old caps or lose transfomer windings. Obviously watch the light or off-load voltage, test before using them as often a 12V 1A linear may be 16-18V lightly loaded.
    4. These need to be high, mainly to get away from noise, I really struggle to use them here, at least they are small enough to move around and try to get away from the QRM
    5. I used as many grounds as I could but had most success seeing one part of the whip/feed as the "antenna" then everything after that needs to be grounded/shielded/buried
    6. Voltage as long as you don't go over voltage I have seen surprisingly good results in noisy locations dropping it back, I even ran the current mag lops under "recommended" voltage range by about three volts seeing only marginal reduction in WSPR spot counts.

    My experience is mostly through trial and error, the link below leads to some more insightful posts from others who have a better understandings of electronics and RF theory.

    I did put an image on this forum on the layout I settled on before switching to LZ1AQ loops, If I find it later I'll link.
    Mine was roughly 7m fibreglass mast, down to earth rod (lightning discharge) and air coil, low pass filter, cctv balun -CAT 5- cctv balun, earth rod, house Bias Tee with filters on the mains feed to linear psu.


    * I am now "older" and have mostly draws of full stuff I "may use one day, too good to throw out".
    cx8bit
  • edited December 2019
    @Powernumpty !

    1 and 4) I'll move the antenna to a higher location and try to use RG6 (shielded 75ohm cable normally used for satellite television) to try to reduce the feed line noise.

    2) I haven't used a magloop yet. The university of Twente uses a Mini-Whip near a city and they reached a decent SNR.
    (http://websdr.ewi.utwente.nl:8901)

    3) Good, I'll try a linear power supply, even though I think this one is filtered since is laboratory equipment.

    5) Right, I can't use more than one ground point since I don't have ground on the roof. Maybe I can add a copper rod to my current setup.

    Another question: Do you think using quad shielded RG6 is a good idea? If the coax is part of the antenna, I'm thinking that too much isolation could hinder the reception.


    Thanks for your detailed response! What is you kiwi url?


    73,
    CX8BIT
  • Sorry for slow reply.
    My wiki is on sdr.hu under M0AQY but note it is set up more for digital weak signals like WSPR, than voice, there is also terrible noise from one long term source.
    Check http://graphs.wsprdaemon.org/M0AQY/ if there is 20db extra noise on 12m don't bother (like there is as I write).

    On the "quad shielded" coax - IMHO see it as two parts.
    Part One is the feed closest to the receiver and any noise, that should either have large ferrite rings OR quad shielded fitting both is a waste, the shielding reduces the affect of ferrite.
    The second part is furthest from the noise/rx and connected directly to the antenna, try different coax types, lenghts and orientations on that. I found that made a dig difference but was hard to know how the interactions would work at each change (part of the reason I gave up on small whips).

    In the link to previous posts Martin G8JNJ and others give a better explanation on the mechanisms associated with these whips.
    You probably have to use a ground reference rather than "earth" which won't be true RF earth on a roof.
    I'd move the whip around real-time while looking at the Kiwi display, looking for the largest sources of local noise, then how far you can get away from it or any structure that re-radiates the noise. Here I have to use the mains on one side of the house as all manner of junk is carried on the noisy side.
    Once you have mapped out the noise it may be possible to put the whip some distance from the biggest source, ideally with something metal or a screen between but watch the feed run too.

    The great advantage of the Kiwi is that you can use a mobile or tablet and move the antenna while watching the spectrum and waterfall. In a built up area it is hard to get away from all noise but you should be able to recognise the sources individual fingerprint (spectrum, waterfall or AM audio).

    I first got into SDR through the Twente WebSDR (try to beat them on wspr spots now) so I believe you should be able to find a way of making it work on a roof.
    I think theirs is a large metal roof or they have enough solid metal railings to act as an elevated RF plane so you could try to replicate that idea after mapping the noise.

    Cheers
    Stu M0AQY
  • Just a couple of additional thoughts following on from Stu's post.

    Both techniques of screening and choking are important, but they perform different functions and have to be applied appropriately.

    "Quad shielded" coax can be many things, but these days many of them are just marketing hype.

    In the past coaxes with multiple screens that were insulated from each other were used for video distribution in TV studios, where it was necessary to separate 'clean' and 'dirty' electrical grounds from each other, so that very low level noise didn't get introduced into the analogue DC to <10MHz video circuits.

    Quad shielded coax as referred to these days, tends to be a generic term that means it incorporates other cores around the inner, and some of these may simply be to provide additional mechanical strength or protection, such as steel reinforcing strands.

    It's best to avoid coaxes that have a plated steel cored center conductor and also those with aluminium foil screen or aluminium conductors.

    All of these can cause problems on the lower frequencies where the conductor skin depth is not adequate for the purpose on the LF bands.

    Common mode noise is carried on the outside of the coax screen, and so as long as the skin depth of the conductor is sufficient, it will remain there and any additional screening won't make much of a difference.

    The problem arises at the ends of the coax where it is terminated, and makes a transition from balanced to unbalanced due to the current on the outside of the outer screen.

    Common mode chokes are required to reduce the amount of common mode current that can be carried along the outer of the screen and can be re-radiated to be received by the antenna. It can also be directly re-introduced into the balanced signal path of the coax by incorrect termination of the screen at that transition point.

    The frustrating thing about common mode chokes is that it's a bit like playing 'Whack a Mole' as soon as you put a choke in one place, the current distribution changes and the noise can pop up on a different frequency. Some degree of experimentation is required to find the best placement, and this often changes when you connect new items of equipment, or even in some cases when you simply plug another item of electrical equipment such as an extension lead, and the long mains cable modifies the RF current distribution on various surrounding structures.

    I have used an E-Probe mounted above the metal roof of an industrial building full of various items of consumer electronics and server farms. The trick is to use the metal roof (and frame of the building) as the ground plane for the antenna, but to ensure that the coax has lots of common mode chokes fitted on the 'building side' of the ground plane connection, so that noise from within the building is not introduced onto the antenna itself.

    Regards,

    Martin - G8JNJ
    Powernumpty
  • quad shield = "we found a way to make coax cheaper and it uses multiple shielding methods"
  • Hello!

    Here are my tests results with different cables:

    RG6 (quad shielded) using heating pipes as GND:


    RG58 (cheap) using heating pipes as GND:


    RG58 (cheap) using a copper rod as GND:


    Conclusion:
    When I installed the RG6, the noise went up and I lost reception (measured with beacons). Looks like the cable is part of the antenna system and plays an important role by receiving signals.

    What do you guys think?
    73!
  • Next time you plot it, use the spectrum display instead of the waterfall so that relative levels can be discerned.
  • edited December 2019
    As I said before, the coax forms part of the antenna so separate it into two sections if you can.
    1. the part closest to the antenna, that part has the largest affect on signal
    2. the part closest to the receiver, that has the most affect on local noise pickup

    If your run is short then it is harder to split the two but where you have say 10m of coax 5m could be decent shielded (grounded to other items) but the second (antenna) half RG58 or similar (yes I know different impedance but it worked for me).
    Everything inside of the join (+ground) should be focused on noise reduction and reducing common mode noise getting back to the receiver or out to the probe, outside is part of a dynamic antenna.

    I'll second the spectrum comment and add turn on "Peak" slow the waterfall too so that you have a longer window, refresh the page and keep it running for the same amount of time for the peak hold.
    Alternatively click the green arrow to copy the link and keep that as a bookmark so tests are comparative.

    --edit--
    On reflection your RG58 noise is not too bad so the above would only be worth trying if you had bad local pickup or wanted to test the dynamics
  • Hello,
    @Powernumpty , yes, got it, the problem is that I run most of the cable vertically and close to the antenna. My main issues are the MW transmitters a few kilometers away (18km).


    From LF to 160m it's all unusable due to broadcasters harmonics. Near 50 hz I only hear switching noise from the power company transformers.




    I'm thinking on installing a HPF that attenuates everything below 2.5 Mhz. What do you think? Will the sdr be able to hear better on other bands if I add that filter?

    73!
  • IMO, there's something more than MW overload going on. I live in a MW "swamp" and don't see that, but I don't have a mini-whip. Is the amp in the antenna going into compression...?
  • The cluster around 25 MHz looks like it may be the 60 kHz LAN family and is an indication of insufficient common mode rejection, not surprising for an unbalanced antenna and coax. If this is conversion from common mode current on the coax, possibly injected by the LAN, I doubt that an HPF will help that at all.
    The closer spacing stuff does look like IMD in a non-linear stage, perhaps the preamp, though there doesn't seem to be anything in-band big enough to account for it. What happens when you turn power to the preamp off?
  • edited December 2019
    Hello!
    When I listen those signals in 160m in AM, sounds like a MW stations pile up (IMD). When I turn the antenna amp off this is what happens:



    and this is what happens near the MW broadcasters (note, I'm using a MW attenuation filter):



    What do you guys suggest? Could it be that strong MW stations are overloading the antenna amp? How could I avoid this?
    Do you think the antenna is not working right?

    Here is the antenna schematic:





    Regards,
    CX8BIT
  • edited December 2019
    Hello,
    I just read that intermodulation is a weak point for this kind of antennas:

    https://www.sdrplay.com/community/viewtopic.php?t=1138&p=6789

    Here I found better designs based on the experience of using a mini-whip in the university of Twente:

    http://www.pa3fwm.nl/projects/miniwhip/

    Here are those ideas implemented:

    https://www.dl4zao.de/active_antenna/index.html#a1846

    Regards,
    CX8BIT
  • I haven't analyzed the IMD of your cascode MOSFET and follower but from your screen shots I think it is clear that you have almost no common mode rejection at upper HF. The transformer you are using probably has no more than a very few dB inter-winding isolation by 30 MHz. Thus current introduced either at the antenna or back on a LAN or power supply cable has a low impedance path and without balance at the preamp ends up coupled back into the kiwi. The family of lines with peaks at 20 and 26 MHz are earmarks of this.

    Your preamp has no balance and probably not a lot to recommend them for strong signal handling. I'd not expect much out of the FET though the 2n5109 is not a bad device and used in push-pull can possibly drive low impedance cable. But of course that does no good if the distortion is created in a prior stage.

    On top of that problem, imbalance at the antenna - in the image antenna which is made up of coax, imperfect grounding etc - means you also get currents from other coupling mechanisms input to the preamp. The coax/power return is just as much your antenna as the whip is and that part of it probably has near-field coupling further complicating things.

    Among all these various mechanisms, I'm not too surprised to see the results you experience.

    While it is very difficult to choke/shunt unwanted currents coming in at the antenna, without a fairly major change doing that and moving the antenna further from near-field sources while improving isolation transformer(s) (maybe two in tandem) are probably the only practical change. A Guanella balun on all lines going into the kiwi might pick up some improvement.
    .
    My own preference is for balanced antenna structures, higher current, lower distortion preamps and balanced transmission lines with carefully controlled baluns designed to work over multiple decades, but even then the problem still has "whack-a-mole" aspects. Like peeling an onion, you identify and remove layers of problems as you go - quitting when you're eyes are crying too much. :)
  • edited December 2019
    Looking at the spectrum display I don't think it's overload from AM broadcast stations, as they are all below -40dB which suggests that the amplifier and KiWi are both well below the overload threshold.

    The spikes seem to occur at intervals of approximately 33KHz which suggests that it's noise from a switched mode power supply, rather than broadcast stations which are spaced at 9 or 10KHz intervals.

    The signal strength also makes me think that it's probably something in your own home.

    EDIT - Maybe even the power supply you are using to feed the bias tee, as I notice that it has got very little filtering on the DC input. Adding a 1000uF capacitor across the existing 470nF on the DC input may help a bit, but it really needs another 470uH inductor on the DC input before the 470nF capacitor to be really effective.

    Something like the right hand side of this circuit, which uses slightly different component values.

    https://www.g8jnj.net/Active monopole antenna DC to 500MHz v1.6.png

    A battery powered AM transistor radio with built in ferrite rod antenna would probably help you track it down, but the first step to try is to switch off your mains supply at the main breaker and see if the noise goes away.

    Regards,

    Martin - G8JNJ
  • His comment
    "When I listen those signals in 160m in AM, sounds like a MW stations pile up (IMD). When I turn the antenna amp off this is what happens:"
    leads me to think it is not a family of lines from a switcher...
    Worth studying this though.
  • Hello!
    Thanks for the response. My knowledge of electronics it's pretty limited right now.

    Based on your comments, I've identified 2 problems:
    1) Intermodulation from the antenna amp near 160m.
    2) Interference coming from the Ethernet cable.

    Problem 1 - is hard to tackle since the antenna doesn't seem to be designed to be near high power MW transmitters.
    I'm thinking of using another antenna design. How about this one?

    https://www.ebay.com/itm/Active-dipole-mini-whip-antenna-VLF-LW-SW-for-RTL-SDR-Degen-Tecsun-Sony/223618775783?hash=item3410b7bae7:g:xuwAAOSwr-ZdTqyz

    Problem 2 - Do you adding ferrites to the lan cable could solve the problem?

    @G8JNJ remember I'm using a MW broadcasters bandstop filter to avoid OV in the MW section.

    Regards,
    CX8BIT
  • what type of PSU do you run to the bias tee (excuse me if this has been asked or stated before)
  • Hello. Don't worry. Right now I'm using a linear 5v power supply.

    Regards,
    CX8BIT
  • Hi,

    >
    >remember I'm using a MW broadcasters bandstop filter to avoid OV in the MW section.
    >

    Yes OK, however the 'IMD' seems to be occurring at intervals of something like 33KHz spacing.

    If it was from MW broadcast stations I'd expect it to be multiples of 9 or 10KHz which is the usual channel spacing.

    However you can try this.

    In the RA0SMS circuit place a 1K Ohm resistor across R1 (1-2M Ohm), this will reduce the amplifier input impedance and cause the frequency response to roll off at the LF end. You may need to experiment with the exact value of resistor to get the amount of roll-off to suit your location, but it will at least help prove if it's a problem associated with the high level of broadcast stations, or if it is something else.

    Regards,

    Martin - G8JNJ
  • If redqueenuy.ddns.net:8073 is the kiwi in question, it looks me like a family of 20 kHz lines, probably BJT at that frequency, SMPS lines with 2nd harmonics lines of 20 kHz visible but only at the low end of the kiwi. It's remarkably close to 20 kHz which may be a clue. Not likely a free running switcher. By upper MW it has pretty much fallen away. The 80 kHz line is showing odd-50 Hz (mains related?) sidebands. Is Uruguay 50 Hz? It looks like full-wave rectifications of the 20 kHz at 40 kHz generating terms which are escaping the output of the SMPS.

    Above MW things are different. These are showing only odd harmonics of 20 kHz which would be expected if both 20 kHz edges were involved. Also very close so I'm suspecting it's all from the same device. But I'd guess that this is a different coupling mechanism, possibly common mode current getting into the kiwi from the mains/PS/LAN side and flowing out through the isolation transformer which has too many turns to have much rejection at HF.

    I could be very wrong, just my impression.

    None of this speculation may be very useful in finding the actual source but once you do, using conventional divide and conquer technique, it may help a little in figuring out the ingress mechanisms which could help make the kiwi more bullet proof to other sources not yet encountered. The isolation transformer speculation is just one aspect of this.
  • Hello,
    thanks @G8JNJ , I will try the resistor approach.

    @n6gn yes, Uruguay is 50hz. I already tried to replace the power supply with a 9V battery, and the harmonics are still there. If I unplug the antenna, they disappear. I suspect they enter through the antenna / antenna cable. I live in city near transformers, led lights, etc.

    What would be the steps to determine form where are those below MW harmonics coming from?


    Regards,
    CX8BIT
  • I think I might try a battery located at the preamp, powering the preamp, in order to determine whether they are coming in before or after it. Then work on the problem that the results indicate. One possibility is that they are entering via common mode or even differential mode on the PS line. The battery test will eliminate that. Another possibility is that they are common mode on the line between the antenna and the kiwi. Changing the amount of isolation, perhaps by inserting a 1:1 transformer with only 1or2 turns to reduce interwinding capacitance.

    That there is a mains component on it isn't necessarily too important since the switcher's supply may not be well filtered and 100 Hz sidebands would be expected.

    See what happens with the battery test first.
  • edited January 2020
    @cx8bit

    I was assuming that the actual antenna on your mini-whip was of reasonable size. If you are only using a very small antenna 'plate' or similar then you may need to use a higher value of resistance such as 10K Ohms.

    However if you are only using a very small antenna element then it's unlikely that the problem is associated with IMD.

    I still think it's much more likely to be a problem with a local noise source or one of the issues that Glenn has outlined.

    A very easy test would be to simply lie the antenna down on its side, on to the ground.

    Compare the signal level of a strong broadcast station before and after placing the antenna on the ground, and compare the difference between the two.

    Then perform the same test with one of the interference 'spikes'.

    If the change in levels in both cases is the same, the noise is being picked up on the antenna or cable and it is not IMD.

    If the change in levels on the noise 'spike' is greater, then it's IMD, because IMD changes level at a much greater rate than the originating signals.

    e.g. The change in level of the broadcast station is 10dB and the change in level of the noise spike is say 20dB to 30dB (assuming you can still see the signal above the noise floor).

    One other test is to use a short 6" or 150mm length of wire directly connected to the center of the KiWi RF antenna input socket, to determine if you can see the noise spikes on the waterfall. If you can, then the source is something very close to the KiWi, which you should be able to locate by moving the KiWi and wire antenna around.

    Regards,

    Martin - G8JNJ
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